It is known to make a polymeric material which can be inscribed by means of laser light by mixing in a filler which changes color under the effect of energy radiation. If it is desired to keep the costs of fillers low, only a partial amount of filler is mixed in with the polymeric base material (DE-OS 2,936,926). In particular, a high-contrast light-colored inscription in the natural color of the plastic on a black background can be produced on the plastic surface by laser irradiation after incorporating 0.08 to 0.125% carbon black or graphite as filler. The whiteness of the characters can be improved by optical brighteners which are added to the pigments and which are not destroyed by the laser light (German patent No. 3,044,722).
Also known are molded articles consisting of thermoplastics which contain a polymer having aromatic structure, which exhibit very good black coloring due to laser irradiation even without an additive which can change color. However, as the various grades of thermoplastics react very differently, there must be considerable coordination of process parameters, and possibly also of material modifications. It is also important to have color which is adjusted optimally for the laser light and of which the light stability remains in the usual limits for the product class concerned. In the case of large molded articles, restrictions in processing are encountered, depending on the grade of thermoplastic (Kunststoffe 78 (1988), issue 8, pages 688 to 691).
A material which can be inscribed well by laser and which has good thermal stability and stress cracking resistance is obtained by mixing polycarbonate with 10 to 50% of an aromatic polyester (European Patent No. 0,249,082).
Finally, a process for the laser inscription of high-molecular weight organic materials is known in which good inscription properties are achieved by incorporating additives (European Patent No. 0,190,997). In particular, commercially available pigments and/or polymer-soluble dyes are used as coloring additives. An inscription of adequate contrast on the surface of the materials of which the color has been adjusted with these additives can be produced by means of a Nd-YAG laser preferably of doubled frequency (wavelength 532 nm after frequency doubling).
Consequently, according to the prior art, a polymeric composition can be adjusted in such a way that it can be inscribed by laser light, whether by choosing a grade of thermoplastic having good laser inscription properties or by incorporating an additive which changes color under the effect of laser irradiation.
However, in most cases the amount of color contrast which is achieved by known methods is not as high as desired. Accordingly, there is a constant need for additives which can cause significant color changes to occur in the polymeric materials in which it is incorporated. Moreover, it is highly desirable that these additives not deliteriously affect the beneficial physical properties of the polymers.